1. Field of the Invention
The present invention generally pertains to a grommet for preventing water and dust penetration. Such a grommet may be fitted onto a wire harness installed in vehicles, e.g. automobiles. The grommet is then mounted into a hole that is formed by boring into a car body made of, e.g., a steel panel. Such a grommet is usually made of an elastic material.
2. Description of Background Information
Known grommets are formed of a rubber or an elastomeric material. As shown in FIG. 1, such a grommet includes, along its longitudinal axis, a small cylindrical portion 1a, through which a wire harness passes in an airtight condition; a large cylindrical portion 1b having a thick endwall 1c; and an intermediate cylindrical portion gradually flaring from one end of the small cylindrical portion 1a and reaching one end of the large cylindrical portion. The grommet also includes a snap fitting peripheral engagement channel 1d which is grooved in the outer circumferential surface between the intermediate cylindrical portion and the large cylindrical portion 1b having a thick endwall 1c. When the grommet together with a wire harness is inserted into a hole made in a car""s body panel, the circumferential engagement channel 1d engages with the peripheral rim of the hole and thus attaches onto the car""s body panel.
The engagement channel 1d of the grommet 1 includes a first channel wall 1d-1 and a second channel wall 1d-2. The car body panel 2 is bored with a hole 2a adapted to the grommet. The rim zone of the hole 2a thus includes a first face 2b and a second face 2c. The first and second channel walls 1d-1 and 1d-2 are closely fitted with the respective first and second faces 2b and 2c of the car""s body panel 2, thus preventing the passage of water and dust through the hole.
A car""s body panel, to which is applied a grommet, is usually rather thin. To achieve good sealing with such a panel, the first channel wall and the second channel wall are to be brought closer together the farther they extend radially from the common cylindrical axis. FIG. 2 shows a first slanted channel wall 1dxe2x80x2-1 and a second slanted channel wall 1dxe2x80x2-2 formed as explained above. The first and second slanted channel walls respectively form a first circumferential edge 1exe2x80x2 and a second circumferential edge 1fxe2x80x2, which hold the inserted body panel portion from opposite sides.
FIG. 3 shows an example of a die from which such an engagement channel is molded. As can be seen in this figure, the portion of the die adjacent the first slanted channel wall 1d-1xe2x80x2 and the second circumferential edge If is rendered very thin, and forms a neck portion 5a. The neck portion 5a then widens toward a channel base 5b of the snap fitting engagement channel 1d. A grommet to be applied to a 1 mm thick body panel, for instance, may include a neck portion 5a having an opening (L1) of 0.5 mm, and a radial distance (H) of 4 mm, as measured from the channel base 5b. 
However, when a grommet contains such a xe2x80x9cthin-top, wide-basexe2x80x9d channel configuration, the dies that are configured complementarily thereto become very susceptible to breakage, incurring a very short die lifetime.
In view of the above, a main object of the invention is to provide a grommet which can be produced without using such dies having a considerably narrowed neck portion. The grommet thus produced can nonetheless provide circumferential edge portions which hermetically fit with a panel material.
To this end, there is provided a grommet made of elastic material having longitudinal and radial directions. The grommet includes at least a cylindrical portion and a cylindrical end portion with a thick wall, arranged along the longitudinal direction, and is adapted to fit onto a wire harness and to be mounted into a substantially circumferential rim surrounding a hole bored through a panel material.
The grommet of the invention includes a circumferential channel on the outer circumferential surface between the cylindrical portion and the cylindrical end portion having a thick wall, such that the circumferential channel forms a channel base, and first and second channel walls extending substantially radially therefrom. Further, the first and second channel walls respectively form first and second circumferential edges at the radially outermost portions thereof, whereby the former is located radially more outwardly than the latter.
The first and second channel walls are configured such that the first circumferential edge projects beyond the second circumferential edge in the longitudinal direction and the minimum distance between the first channel wall and the second channel wall in the longitudinal direction can be maintained above a predetermined value.
Preferably, the first channel wall includes a circumferential channel extending along the second circumferential edge, and the first and second channel walls are configured, such that they are brought closer together as they extend outwardly and substantially radially from the channel base.
According to a further aspect of the invention, the first and second channel walls may be configured such that the first channel wall is bent at a flex point near the second circumferential edge, whereby the first channel wall extends, up to the flex point, in parallel relation to the second channel wall while inclining towards the cylindrical end portion in the longitudinal direction, and the first channel wall then inclines toward the second circumferential edge from the flex point onwards.
According to a further aspect of the invention, the first circumferential edge may include a flat edge in parallel relation with the radial direction, so as to hold the panel material.
Typically, the minimum distance between the first and second channel walls in the longitudinal direction is about 1 mm, when the panel material is about 1 mm thick.
The portion of the first channel wall near the second circumferential edge, where the distance between the two walls is the smallest, is preferably provided with a groove having a semicircumferential cross-section, so that the above distance is advantageously made greater. Accordingly, the width of the corresponding member in the molding die can also be made greater. As a result, the dies can enjoy a longer service life. Moreover, although the first channel wall is grooved, the first circumferential edge overlaps with the second circumferential edge in the longitudinal direction. Further, the first circumferential edge has a flat edge so as to yield a larger adhering surface, to which a panel material, e. g. , a car""s body panel, abuts. By virtue of this specific configuration, sealing quality is not impeded, even though the channel surface is reduced.
In a typical case, the first channel wall forms a channel section flexing at a predetermined position, up to which the first and second channel walls extend in parallel relation to each other, while keeping a constant width without including any unacceptably narrowed position. An advantage of having such a channel structure is that the dies designed for molding such a channel structure can secure a longer service life. The portion of the first channel wall located above the flexing position is inclined towards the second circumferential edge and overlapped therewith in the longitudinal direction. Moreover, the tip of the first circumferential edge is made flat, so that it can have a larger contact face for holding the panel material. The sealing quality of the grommet can thus be maintained.